The substrate was maintained at fie

The substrate was maintained at field capacity from planting to tuber initiation onset (TIO) which was defined as the stage when tuberous swelling of the main stolons had at least twice the stolon diameter (larger diameter of 10 mm approximately) (Jefferies and Lawson, 1991; Ewing and Struik, 2010) determined by daily harvest of extra pots. After TIO (Table 1), two treatments were applied for each genotype: a control, in which the substrate was maintained at field capacity by replacing every 48 h the water lost by evapotranspiration, as determined gravimetrically; and a water restriction treatment in which the replacement of evapotranspired water was limited to 50% of FC. To avoid a  overestimation caused by the increasing plant biomass, at every harvest we estimated a calibration factor for readjusting  values from pot weight. Thus, we calculated  of a substrate sample from the harvested pots (using Eq. (1) with PW = 0) weighting the moist sample, drying it at 105 ◦C for 24 h and reweighting it. A calibration factor was calculated fitting a linear function between  of the sampled substrate and  estimated by pot weight.